1. Field of the Invention
The present invention relates generally to control systems for hard disk drives, and, more specifically, the present invention provides a circuit for determining and applying an overshoot current (Ios) with a selected amplitude and timing (phase) to a generated square wave current (Isq), which in combination is provided as a write current (Iw=Isq+Ios) to a write head in a hard disk drive.
2. Description of the Background
In a conventional hard disk drive, one or more magnetic media (disks) are written to and read from by magnetic heads. Typically, to write a certain bit of information to a location on the magnetic medium, a write head is moved over the specified location on the hard disk, while a magnetic field is generated via coils in the magnetic head. A write current is then applied to coils in the write head, causing the magnetic field to be circulated through the hard disk medium, thereby storing information. By rapidly rotating the magnetic media mounted on a spindle, and further by rotating an arm of a gimbal with the magnetic head suspended on the end thereof radially across the surface of the medium, the magnetic head can write a large amount of information in a short amount of time.
To enable the write head to determine when to generate a magnetic field to write information to the magnetic medium, a write current Iw is supplied to the write head. This write current Iw is generally a series of current pulses (pulse train) that arrive at the write head at the moment when the head travels over a location on the magnetic medium to be written to. To enable faster writing, the frequency of the pulse train that makes up the write current Iw needs to be increased. However, as the frequency of the pulses increases, the leading edge of the pulse becomes distorted because “real world” electronics are not able to generate a perfect incoming square wave pulse, and further because the impedance of the write head will deform the pulse (as described more fully below).
To at least partially correct these pulse deformation problems, and to allow for a higher frequency write current Iw, an overshoot current Ios is typically added to the generated square wave Isq, the combination of the two currents becoming the write current (Iw=Isq+Ios). The overshoot current allows the write pulses to have an decreased (i.e., faster) rise time so that a higher frequency write current can be used to write information with a write head, thereby increasing the speed at which information can be recorded to the magnetic medium.
FIG. 10 generally shows this combination of currents. In FIG. 10(B), a square wave current Isq is shown as an imperfect square wave generated by a square wave generator. The rising edge of the square wave current is not a perfect vertical line, and the response of the write head to receiving this signal Isq as the intended write current would not be as intended by the system. FIG. 10(A), therefore, shows an overshoot current Ios with a given amplitude (Amplitudeos) that is generated to correct the deformities in the square wave current Isq. FIG. 10(B) shows that the square wave current Isq is combined with the overshoot current Ios after some time delay (Timingos). In other words, the overshoot current is generated, a small time delay (Timingos) is waited, and the square wave current is then combined (via an adder) with the existing overshoot current Ios. The combined write current (Iw=Isq+Ios) is shown in FIG. 10(C) as the result of this addition. The resulting write current includes a faster rise time and will cause the write head to perform more closely to the calculated intentions of the system. The amplitude (Amplitudeos) and the timing or phase (Timingos) of applying the overshoot current are both predetermined and fixed in current systems.
The main problem with this conventional overshoot addition is that the write pulses do not occur at the same frequency, and that the write head will respond differently (e.g., has a different impedance) at these different frequencies. This aspect is depicted in FIG. 4. FIG. 4 shows an exemplary write current waveform showing one complete write current pulse and the front half of a second write current pulse. As seen in the drawing, the initial write current pulse (square wave) has a write frequency of f1 (wherein the period of the pulse itself is only half of this value, or 1/(2f1)). An overshoot current has been added to the square wave current to correct for the distortion caused by the impedance of the write head. The time delay between the initial current pulse and next current pulse has a frequency of f2 (period of 1/(2f2)), and the next write current pulse has yet another frequency f3 (period of 1/(2f3)). Therefore, each pulse (and time period between pulse) may have a separate frequency. The overshoot current amplitude and timing (phase) is not conventionally adjusted to account for these different frequencies.
The multiple frequency problem occurs because the write head has a different impedance at different write data frequencies, and this impedance difference will distort the incoming write data current in different ways (see generally, FIG. 5). Therefore, a single overshoot current of a fixed amplitude and a fixed timing will not properly correct the distortion in the square waveform current at all possible operating frequencies. Instead, the applied overshoot current amplitude and timing of application should be altered with each incoming write current pulse to maximize the chance that the write head will correctly store information in the magnetic medium. Solutions in this vein are desired.